The present invention relates generally to methods, systems, and instruments for inserting prosthetic implants, instruments, and other devices and, more particularly, to methods, systems, and instruments for inserting implants and other instruments into medullary canals of bones.
In arthroplasty, a prosthetic implant is often inserted into a medullary canal of a bone. In addition to the implant itself, other instruments or devices may also need to be inserted. Examples of these instruments include broaches, reamers, alignment tools, depth guides, and gauges. Because of medullary and cortical tissue within the bone, insertion of the implant or instrument typically requires the delivery of a force to the implant or instrument in order to drive the implant or instrument to a desired position within the bone. Various methods, systems, and instruments have been devised to assist in the placement of the implant or instrument at the desired position within a bone.
For example, an inserter or impactor is a commonly used device for driving an implant into the medullary canal of a bone. The inserter features a rod that is in some way connected to a frame and which runs through this frame and connects to one end of the implant, as by a threaded end on the rod which is received in a threaded bore in the end of the implant. Additionally, the inserter features a load transfer surface integral to the frame near the end of the rod, which abuts against an impact surface at the end of the implant. The rod is securely connected to the implant in order to apply compression between the inserter and the implant, and more specifically to compress the impact surface of the inserter against the impact surface of the implant. After the inserter has been firmly secured to the implant, a surgeon applies forces to the inserter as with a mallet and these forces are transferred through the inserter frame to the implant to cause the implant to move to the desired location. After the implant has been driven to its desired location, the surgeon disconnects the rod to remove the inserter from the implant.
A difficulty arises during the impaction of an implant in that the mallet blows cause vibrations within the inserter and these vibrations lead to a retraction of the rod from within the implant. As the rod retracts, the compression between the impact surface on the frame of the inserter and the same on the implant is relieved and the inserter becomes separated from the implant. Thus, the successive vibrations from the inserter result in an unseating of the inserter from upon the implant. Once the inserter becomes separated from the implant, the inserter no longer efficiently transfers forces from the mallet blows to the implant. Instead, the mallet blows may result in undesirable forces being applied to the inserter or between the inserter and the implant which adversely effect the integrity of the inserter, the implant, or the surgical procedure.
For example, retraction of the rod from within the implant may damage the rod itself If the inserter is not seated upon the implant, then some of the forces from the mallet blows are diverted to the rod. The rod, however, is designed to impart compression between the inserter and the implant and is generally not designed to receive impact loads and transfer these impact loads to the implant. Because the rod is not designed to withstand these loads, the impact forces cause damage to the threads, such as by galling, stripping, or flattening the threads. This damage to the threads of the rod may prevent the rod from being completely retracted from the implant, in which case the implant must be removed and replaced with another implant. In addition to damaging the threads, the mallet blows delivered to the inserter may present a sufficiently large enough stress to fracture the rod.
The inserter and implant may also become damaged if the rod retracts from within the implant. For example, as the rod retracts, the load transfer surface on the inserter can become spaced from the implant. This spacing may prevent the load transfer surface of the inserter from being properly aligned with the load transfer surface on the implant. The offset in load transfer surfaces can cause unequal loading within the surfaces resulting in stressing, chipping, or deformation of these surfaces. Because of a spacing between the implant and inserter, the impact forces may also place the inserter in motion relative to the implant and create a collision between the two members. The motion imparted to the inserter aggravates the potential for damage to the inserter or implant.
Besides causing damage to the implant and inserter, a retraction of the rod from within the implant during the impaction process complicates the surgical procedure. If any damage occurs to the rod, other parts of the inserter, or to the implant, the insertion of the implant must be halted and the surgeon must investigate the extent of the damage. The possibility exists that the implant must be removed and the surgeon must repeat certain steps of the procedure. At the very least, damage to the rod, inserter frame, or implant presents a delay which increases the time required within the operating room and necessarily the costs involved in the surgery.
More importantly, damage to the rod, inserter frame, or implant is another complication in the procedure and risk to the patient. For instance, if the rod breaks during the insertion of the implant, the implant having the broken-off portion of the rod must be removed. A retracting mechanism that is typically used to remove the implant, however, also has a rod for insertion within the bore of the implant. The retracting mechanism therefore cannot be readily used to remove the implant. The implant consequently cannot be easily removed from the patient.
The mallet blows directed to the inserter can quickly and unknowingly cause damage without any warning. The retraction of the rod from within the implant during impaction does not follow a linear pattern. Instead, a loosening of the rod promotes additional loosening and compounds upon itself quickly escalating toward possible, probable, or enhanced probability of failure of a component. Thus, even though a surgeon may have fairly recently tightened the inserter to the implant, the rod can retract after only a few blows with the mallet thus causing damage.
It is therefore imperative that forces delivered to an inserter are efficiently delivered to the implant and are not diverted or otherwise transferred to the rod itself. A proposed procedure for ensuring proper seating of the inserter on the implant involves periodically tightening the rod within the implant. A goal of the procedure is to tighten the rod before it becomes too loose with the hopes of preventing any damage from occurring. The proposed procedure involves tightening the rod after every certain set of blows with a mallet, such as after four or five blows.
The proposed procedure in which the surgeon stops and periodically adjusts the inserter is not optimal. For one, retraction of the rod from within the implant is often unpredictable whereby selecting one number may be useful, but may not prevent all damage to the rod, implant, or inserter. Further, the procedure interferes with the surgeon""s ability to quickly place the implant at the desired location. Most surgeons prefer to focus on the insertion of the implant and having to stop and tighten the inserter after every four or five blows is a burdensome distraction. A need therefore exists for an inserter that reduces the possibility of damage to the rod, inserter frame, or implant and is better able to remain seated firmly on the implant.
The present invention addresses the problems described above by providing methods, instruments, and devices for moving objects such as prosthetic implants or instruments to a desired position. An insertion device according to a preferred embodiment of the invention includes an engagement member, such as a rod, that is inserted into an object. The engagement member is contained within a member for transferring forces to the object, such as a frame, that has a load transfer surface, such as feet, for engaging and resting upon the object. In general, a pre-load compression is applied between the inserter and the object to ensure proper seating of the inserter upon the object. This compression ensures that impact forces are transferred to the object and are not diverted to the engagement member or to other portions of the inserter.
In the preferred embodiment, the inserter includes a threaded rod which is inserted into the implant or instrument and a tension is applied to the rod. According to one aspect of the invention, the inserter has a lever which, when closed, results in a resultant force between the rod and a frame of the inserter. This resultant force directs the inserter frame onto the implant to firmly seat the inserter on the implant. The lever has a cammed surface which moves a slider along a length of the rod when the lever is closed. At least one biasing member is positioned between the slider and a thumbscrew which is engaged to the rod. When the lever is closed, the slider is moved closer to the thumbscrew and the biasing member becomes compressed. The biasing member, in turn, opposes these compressive forces, resulting in forces directing the frame of the inserter toward the implant. The biasing member preferably comprises spring washers but may comprise any component, device, or system that applies compression between the inserter and the implant as impact forces are applied to the inserter.
Because compression is applied at all times between the inserter and the implant and tension is applied to the rod, the rod is substantially prevented from becoming loose, thereby minimizing any retraction of the rod from within the implant. Further, even if the rod does retract, the inserter nonetheless remains seated on the implant as long as the distance that the rod retracts remains within the range of the biasing member compression distance. The compression that is applied between the inserter and implant reduces the risk of damage to the rod, other parts of the inserter, or to the implant and thus reduces complications and costs associated with the surgery. On the other hand, inserters without the biasing members have been known to unlock during use.
Accordingly, it is a feature of the present invention to provide methods, systems, and instruments-that efficiently transfer forces to objects such as prosthetic implants or instruments.
It is another feature of the present invention to provide methods, systems, and instruments that are firmly seated on the prosthetic implant or instrument.
It is a further feature of the present invention to provide methods, systems, and instruments that reduce damage to the inserters, the implants, or other instruments.
It is still a further feature of the present invention to provide methods, systems, and instruments that are easily operated.
It is yet another feature of the present invention to provide methods, systems, and instruments for use in positioning implants or instruments at desired locations.
It is still another feature of the present invention to provide methods, systems, and instruments for focusing forces to the implants or instruments.
It is another feature of the present invention to provide methods, systems, and instruments having threaded rods that do not easily become inadvertently unthreaded during use.
It is another feature of the present invention to provide methods, systems, and instruments that do not easily become unseated.
Other objects, features, and advantages of the present invention will become apparent with respect to the remainder of this document.